Heat treatment of metals



' Dec. 18, 1928.

A. S. MACDONALD HEAT TREATMENT OF METALS Filed Oct. 11, 1924 @WJW A 'I'TORNEYS' a factor in the results.

-cooling.

Patented Dec. 18, 1928.

UNITED STATES PATENTOFFICE.

ANGUS s. MACDONALD, or GREAT NECK,

NEW YORK, ASSIGNOR 'ro SNEAD & com- PANY, A CORPORATION OF NEW JERSEY.

HEAT TREATMENT OFMETALS.

Application filed October 11, 1924. Serial No. 742,966.

,as regards both chemical condition and physical structure,and the consequent character and properties of such product; and to assure uniformity in the results obtained on different occasions or by different operators. Other advantages obtainable through the 'invention include relative quickness of operation-and reduction in handling of the objects or pieces treated; simplicity, convenience, and relatively low cost; flexible and accurate control over the operation in all its phases; and minimization of the personal equation as How these and other objects and advantages can be realized through the invention will appear from the description hereinafter of selected and preferred modes of practicing it.

I have discovered that the results of heat treatment can be regulated and controlled by the manner of heating, in conjunction with quenching, or cooling incident tothe treatment. For the best results, it is necessary not only to conduct the heating in such wise as tobring the material to the right crystal-' line structure and to produce in it the proper chemical condition or inter-relation of constituents, but also to correlate the cooling with the heating in such a way as ultimately to retain or realize the desired structure and chemical composition in the material after With such correlation, the combi-' nation of carbon and iron in steel (for example) can be given the character of austenite, martensite, troostite, sorbite, or pearlite, as may be desired; and the crystalline structure can be made fine or coarse, regular or hap hazard, at will. When heating is effected electrically, by passage of current through the piece under treatment, the ultimate constitution of the material can be very largely controlled by suitable determination of the kind, intensity, and frequency of'the electric current.

In this various effects of the electric current itself on the material are a factor, as wellas the fact that by current passed through it the'piece can be heated uniformly throughout its mass and brought to exactly the tempera ture desired either slowly or quickly, according to the schedule of energy input observed. For example, the general agitation of the crystals produced by electric current (and especially by alternating current) renders them more mobile in their formative state, favors uniformity of grain growth, and tends toward a more stable ultimate, final arrangement of crystals. As a result, internal strains in the material are minimized, and its strength and reliability correspondingly improved. Moreover, some iron-carbon combinations, for example, are magnetic (e. g., martensite) and if the crystals of such a substance are subjected to the influence of electric current passing through the material while they are being formed and their positions are becoming fixed, a more orderly and regular arrangement of the ultimate crystals is brought about,just as fine iron filings placed near a magnet assume a definite, orderly arrangement that isgetgrmined by the direction of the magnetic In explanation of the fact that fine grain structure and crystalline arrangement are more favorable to the strength and other desirable qualities of metal'than coarse crystallization and haphazard granular arrangement, it is to be remembered that the strength of metals and alloys (such as even plain carbon steel) is in part due to a kind of inter-crystal line cement which surrounds the individual crystals and is of greater strength than the crystals themselves,or, at least, lacks the planes of easy cleavage so characteristic of crystals. Such being the structural composi-, tion of steel, it follows that if some crystals are large and others small, and they are all This is so forithe reason that it is not average strength that determines the effective strength of a material, but minimum strength; and because where the crystals are small, the proportion of inter-crystalline cement and the strength of the material are higher. With crystals of small size in an orderly arrangement, therefore, the variation of strength is less, and the effective strength of the material is more nearly the same as its average strength.

As'regards the relation between heating and cooling, the matter of time is of great importance. Even when cooling is gradual, as in annealing, finer grain structure and better mechanical properties can be secured by heating the metal up uniformly throughout its mass to a temperature at-which the recrystallization in which annealing consists will take place quickly, and then stopping the heating at once,'avoiding prolonged soaking at a final fixed furnace temperature and the consequent opportunity for coarse crystallization, especially in the outside of the piece. In the case of sudden cooling or quenching, on the other hand, great advantage can be obtained by making the end of the heating coincide with the application of the quenching medium,'avoiding any intermediate cessation of heating (however brief),.as in the interval required for transferring a piece from a gas-fired mufiie to a quenching bath. In this way, quenching can be effected at exactly the intended tem- .perature every time, and much greater umformity of product attained. To, realize the full advantage of this, however, it is practically necessary that the piece be heated uniformly throughout its mass, just as in the annealing practice above set forth.

The advantages of quenching coincidently with the end of the heating are Well exemplified in the heat treatment of plain carbon steels: steels, that is, without alloy of other metals (such as nickel, for example) that alter its properties as regards heat treatment. Heretofore, it has been found commercially impracticable to secure an austenitic condition of a plain carbon steel; because in such a steel austenite can exist only at temperatures within a very limited range, and is transformed into martensite by cooling during the brief time required to transfer the piece from the heating furnace to the quenching bath. When such steel is heated electrically, and

quenched while current is still passing through it, however, the cessation of heating that gives opportunity for cooling and loss of austenitic condition are avoided; and thus qualities of strength, ductility, and toughness can be secured in plain carbon steels which have hitherto been realized only in steels with alloyage ofother metals that extended the range of temperature in which austenite can exist.

Not only is passage of current through the piece or object at quenching advantageous as 'a means of assuring that the metal has just the right temperature throughout its mass when quenched; it is also the means of realizing in the quenched piece the favorable effects of current on grain structure that I have ex-.

plained above. This is true even in cases where the piece is heated up externally be- .fore current is passed through it: e. g., a

piece may be taken hot from a gas-fired muflie, current passed through it long enough to asthrough and through; and the quenching medium applied before the current is shut off. As a result, the influence of the current on the grain structure is felt at the criticaltime when the structure is being definitely fixed, and its effects thus retained and realized in the final product; whereas if current were shut ofi just before quenching, its structural effect would be largely lost, even if the metal should by good luck be quenched at just the right temperature. v w

The method of electrical heating and sure the desired temperature of the metal a quenching before shutting ofi current is, of

course, favorable'to the limitation and uniformity, of grain size generally necessary to assure the best mechanical properties of the metal, since it allows the metalto be straightway quenched asso'on as it has been'brouglit to a proper quenchingtemperature, without any occasion for prolonged heating or soak-' 111g at a substantially. constant apparent temperature in order to assure that the interior of the piece has attained the desired quenching temperature as well as its exterior. Like- W188, the'accurate control over temperature that is possible with electrical heating allows the piece to be safely brought to'a temperature at which the desired grain structure effect (to be fixed by the quenching) is quickly and simultaneously brought about throughout its entire mass.

In practice, varioustypes of apparatus may be employed formy purpose, such, for example, as those shown in U. S. patents to Macdonald and Huggins No. 1,274,919 granted August 6, 1918 and No. 1,334,663. granted March 23, 1920, or in applications Serial Nos. 639,231 of Macdonald, Huggins and Waite, and 652,047 of Macdonald and lVaitc filed in the U. S. Patent Office-May .16, 1923 and J u'ly 17, 1923, respectively,'and

both assigned to the assignee of this application. Whatever the apparatus employed, p'rovision may, of. course, be made for subj ectlng the piece'to the action of the quenching medium in situ, so to speak;in the clamps or terminals of the machine,as shown, for instance, in U. S. patents to Macdonald No. 1,335,997 and 1,376,106 ,Apri1 6, 1920 and April 26, 1921. In the case of the machine shown in Patent No. 1,335,977, of

. course, the tube for injecting the cooling l/Vhile apparatus such as indicated above is of advantage in obviating all necessity for handling pieces from the time they are initially placed in the clamps to the time they are finally discharged after quenching, my invention can also be practiced very easily and conveniently without any such apparatus, as illustrated in the figure. As there shown, a pair of runways 10, 10 with heat resistant thermally and electrically insulative top facings extend from one side of a bench 11 on which a supply of tubular or other pieces to be treated may conveniently be piled,brass condenser tu es, for example. Two men, A. and B work on opposite sides of the runway, each with a portable terminal clamp 12 having a convenient insu lated handle 13. The clamps 12, 12 are connected to flexible leads 14, 14 from any suitable source of low voltage, high amperage current. such as a constant current transformer 15. In the heating circuitis an instrument 16 for regulating the amount of energy applied, such as a recording watt meter that can readily be reset to zero and also a self-closing switch 17 adapted to be opened by foot pressure: At a. break in the ways 10, 10 is a vat or tank 18 containing the quenching bath,oil or slightly acidulated water, for example.

A and B pull the tubes P from the pile, one by one, to the adjacent sections of the runways 10, 10, fasten the terminal clamps,

12, 12 to each and allow it to heat until a certain amount of energy has been expended on it, as indicated by the watt-meter 16. and then plunge the tube into the quenching bath. In a second or so, A opens the switch .17 with his foot; the tube P is lifted out of the bath and the clamps 12, 12 detached; and the tube is rolled on downthe other (sloping) section of the Ways 10, 10 to any desired point of delivery or reception. Thereupon the instrument 16 is reset to zero and the operation repeated. If desired, of course, the

length of time the tube remains in the bath before the heating circuit is opened may be determined by a second reading of the wattmeter 16.

It should be noted that in heating with electric current without soaking, the danger of burning steel is greatly reduced, as there is no time for oxygen or other deleterious gases to penetrate the heated pores of the metal. Likewise, such a deleterious component of steel as phosphorus has not time to vitiate the inter-crystalling cement by form ing an alloy envelope therewith around the crystals,-which tends to destroy the strength of the metal. On the other hand, carburizing or case-hardening can be carried out more advantageously with electric heating, since the current puts the metal into a condition favorable to effective penetration by the carbon. Thus the time required is less. internal strains in the metal are obviated or relieved, and the quality of the produce is generally improved.

I claim:

1. A method of heat treating metal which comprises heating the body of metal to be treated, by passage of current therethrough. up to a temperature at which the desired grain structure effect is quickly brought about, and thereupon quenching it before shutting off the current. I I

2. A method of heat treating metal which comprises heating, up the body of metal to be treated, by passage of current therethrough, until the desired chemical composition and grain structure of the metal are produced, and thereupon straightway subjecting it to a quenching medium. leaving the current on until the composition and grain structure have become fixed by the quenching.

In testimony whereof, I have hereunto signed my name.

ANGUS S. MACDONALD. 

